In the field of optical communication there is a strong demand for optical shaping and reshaping of optical signals to achieve high speed transmission of optical information at a very high quality and very low Bit Error Rate (BER).
The implementation of ultra fast optical communication network faces, among other challenges, a need to maintain high quality optical signals along significant distances for keeping very low BER. At high transmission rates, the pulse quality of the optical pulses degrades very rapidly in a relatively short distance due to pulse broadening caused by CD and PMD.
The broadening phenomenon of optical pulses, in radiation guides, caused by CD is due to the dependency of the propagation speed on the wavelength. The longer is the wavelength the faster is the propagation speed. The optical pulses have a spectral width Δλ of wavelengths-around the central peak wavelength λcenter. Each wavelength in the spectrum of the wavelengths, related to the optical pulses, propagates at different speed, resulting with pulses broadening. The amount of broadening depends linearly on the traveling length L, the relation between the wavelength and the refractive index described by the slope factor K, and the spectral width Δλ of the pulses.
The broadening process of optical pulses in radiation guides caused by PMD is due to the dependency of the propagation speed on the polarization orientation. Due to production imperfections, the optical fibers have fast and slow propagation axes that are orthogonal. The polarization vector of the optical pulses may have components along the fast and the slow axes. Accordingly, each component of the polarization vector propagates at different velocity and results with broadening of the pulses. In addition, temporal environmental influences may affect the orientation of the fast and the slow axes and may cause changes in the pulses broadening and in the polarization orientation of the pulses.
The PMD dispersions may include first and second orders dispersions corresponding to the broadening of the polarization modes by chromatic dispersions and depolarizations, respectively.
The broadening of the pulses may cause adjacent pulses to overlap each other such that they cannot be resolved for the purpose of information reading. This process is known as Inter Symbol Interference (ISI). The broadening limit of the pulses is the maximum width of the pulses in which the BER exceeds a certain upper limit allowed. The broadening limit for CD and PMD is about 20% and 10%, respectively. In addition to the BER, there is another criterion of measuring the signal quality known as power penalty. The signal quality and the power penalty are determined according to the increasing power factor (measured in dB) needed to be used in order to restore the detection quality (BER) of signals corrupted by dispersions and to bring it into the detection quality of undistorted signals or signals with a certain detection level determined by a certain BER.
Accordingly, if no correction is used to compensate for the pulse broadening caused by the CD and the PMD, many Optical-Electrical-Optical (OEO) regenerators should be distributed along the propagation path in order to regenerate new narrow pulses wherever the broadening of the pulses exceeds the limit that the system can tolerate. OEO regenerators are very expensive and complicated and thus dramatically increase the network cost in terms of infrastructure initial cost and maintenance cost. In addition, the OEO regenerators reduce the network reliability.
There are several methods and techniques designed for CD compensation based on a fiber that produces a process that is opposite to the CD process, i.e., negative CD. According to these techniques, the compensation fiber creates a process in which the longer (and faster) wavelengths are delayed with respect to the shorter (and slower) wavelengths. The length of the compensation fiber is adjusted to produce the compensation needed for bringing the optical pulses back into their original width. However, when such CD compensation may be effective around a certain wavelength, it is very hard to produce dispersion management in which the CD compensation is effective for multiple wavelengths such as used with Dense Wavelength Division Multiplexing (DWDM). Thus, usually the CD compensation is not effective for all the wavelengths used. In addition, even where the CD compensation is effective, these methods provide local correction to the CD, but they do not contribute anything for decreasing the CD effect along further propagation post to the CD correction.
The problem that PMD creates becomes dominant at bit rates above 10 Gbps. At such bit rates, the accepted amount of broadening is normally less than 10%.
Compensating for PMD is more complex than compensating for CD due to the manifold of parameters (like temperature, small imperfections of the fiber, etc.), which may change over time and interact in an unpredictable way, resulting in an inherent randomness of this phenomenon. In addition, PMD does not scale linearly with the traveling length L, but with its square root.